reality provides us with facts so romantic that imagination itself … · 2007-12-26 · sojourner...

Reality provides us with

facts so romantic that imagination itself

could add nothing to them.

Jules Verne (1828–1905)

Mystery

10

Is There Life Elsewhere in Our Solar System?

Science fiction writers once wrote epic stories of civilizations on Mars. Vi-

sions of a Martian civilization have long since faded, but today scientists are

debating whether a meteorite holds fossil evidence of microbial life on Mars.

In this mystery, we will discuss the prospects for finding life elsewhere in

our solar system, with particular emphasis on the cases of Mars, Europa (a

moon of Jupiter that is believed to have a subsurface ocean), and Titan (a

moon of Saturn that is rich in organic compounds).

A century ago, belief that a civilization lived on Mars was so widespreadthat the term “Martian” became essentially synonymous with “alien.”Although occasional speculation about life on Mars goes far back intohistory, the craze began in 1877, when Italian astronomer GiovanniSchiaparelli reported seeing linear features across the surface of Marsthrough his telescope. He named these features canali, the Italian wordfor “channels.” English accounts mistranslated the term as “canals,”and coming amidst the excitement surrounding the recent opening ofthe Suez Canal (in 1869), Schiaparelli’s discovery soon inspired vi-sions of artificial waterways built by a Martian civilization.

Schiaparelli himself remained skeptical of such claims, and it’s notclear whether he even thought the canali contained water. He maysimply have been following a long-standing tradition adopted for theMoon, where visible features were referred to as bodies of water. Buthis work caught the imagination of a young Harvard graduate namedPercival Lowell.

9

Lowell came from a wealthy and distinguished New England fam-ily. His brother Abbott became president of Harvard and his sister Amywas the well-known poet. After a few years as a businessman and as atraveler in the Far East, Percival Lowell turned to astronomy. Impas-sioned by his belief in the Martian canals and enabled by his wealth,Lowell commissioned an observatory (the Lowell Observatory) inFlagstaff, Arizona. Upon its completion in 1894, he set about makingregular observations of Mars. In 1895, he published detailed maps ofthe canals, as well as a book expounding not only his belief that theywere constructed by a Martian civilization but also his conclusionsabout the nature of that civilization. Because Lowell had correctly de-duced that Mars was generally arid and had icy polar caps, he imag-ined that the canals were built to carry water from the poles to thirstycities nearer the equator. (He was incorrect in assuming the polar capsto be water ice; they are largely carbon dioxide ice.) From there it wasa short step to imagine the Martians as an old civilization on a dyingplanet, and the global network of canals convinced Lowell that theywere citizens of a single, global nation. Such ideas provided the “scien-tific” basis for H. G. Wells’s The War of the Worlds, published in 1898.

The canal myth persisted for decades, despite the fact that Lowell’scanals never showed up clearly in photographs and other astronomersdid not see them. Belief in Martians remained widespread enough tocreate a famous panic during Orson Welles’s 1938 radio broadcast ofThe War of the Worlds, when many people thought an invasion wasactually under way. The debate about Martian canals and cities wasnot entirely put to rest until NASA’s first two spacecraft to Mars—theMariner 4 flyby in 1965 and the Mariner 9 orbiter in 1971—sent backimages of a barren, cratered surface.

Today we can be quite certain that there is no great Martian civi-lization, and probably never has been. But the possibility of microbiallife on Mars remains a hot topic in science. Moreover, several otherworlds in our solar system now seem like possible homes to indige-nous life. Thus we are led to Mystery 10 on our list: Is there life else-where in our solar system?

10 ON THE COSMIC HORIZON

The first step in approaching this mystery is deciding just what itis that we’re looking for. Even on Earth, biologists are not quite surehow to define life. Plants and animals and bacteria are all clearly liv-ing, but what about viruses, which cannot even reproduce unless theyinvade some other organism? If we tried to imagine every conceivableform that life might take, the list might well be endless. We cannot ad-dress such infinite possibilities all at once, so the best way to approach

11MYSTERY 10: IS THERE LIFE ELSEWHERE IN OUR SOLAR SYSTEM?

(Above) This photograph shows a working model ofthe Viking landers, identicalto those that landed on Marsin 1976. It is on display at the National Air and SpaceMuseum in Washington,D.C. (Left ) This pair of be-fore and after photographs,transmitted back to Earth bythe Viking 2 lander, showswhere the robotic armpushed away a small rock on the Martian surface.

this mystery is by searching for “obvious” life—life that bears enoughresemblance to life on Earth that we will know it when we see it. Thissearch began when Viking 1 landed on Mars on July 20, 1976.

Mars is an appealing place to look for life, because it is the mostEarth-like planet in the solar system, at least on the surface. Neverthe-less, today’s Mars would not be a very comfortable place to visit. Theatmosphere is far thinner than that on the top of Mount Everest, andno human could survive more than a few minutes without a pressur-ized spacesuit. The temperature is usually well below freezing, andthere is no liquid water anywhere. And because the atmosphere con-tains only trace amounts of oxygen and hence essentially no ozone(which is a form of oxygen), the Sun’s deadly ultraviolet rays pass un-hindered to the surface. Any Earth-like organism that left itself ex-posed would be quickly killed by this ultraviolet radiation.

But photographs of Mars taken by orbiting spacecraft reveal un-mistakable evidence of a very different past—dried-up riverbeds, vastfloodplains, and perhaps even a huge ocean. Clearly, water once flowedon the Martian surface, and that could have happened only if Marsonce had a much thicker and warmer atmosphere. Although other ge-ological evidence suggests that any oceans dried up at least 3 billionyears ago (but photographs from Mars Global Surveyor suggest thatsome water flows may be much more recent), we have no reason todoubt that conditions on the young Mars were nearly as hospitable tolife as conditions on the young Earth. Because life on Earth was al-ready flourishing more than 3.5 billion years ago, it’s easy to imaginethat the same thing occurred on Mars. If so, Martian life may havefound ways to survive—perhaps underground or in the icy polarcaps—even as Mars itself became a cold, dry desert.

Viking 1 and its twin, Viking 2, were each equipped to search forMartian life in a fairly simple way. (Each was also accompanied on itsjourney to Mars by a Viking orbiter, which studied Mars from above.)Like all planetary missions to date, the Viking spacecraft made onlyone-way journeys. Observations and experiments designed to searchfor life had to be planned before the spacecraft were launched andwere carried out robotically by the spacecraft on Mars. The spacecraft


then transmitted pictures and otherscientific data from Mars back toEarth. The Viking landers couldnot move about the surface at all,but each lander had a robotic armthat it used to push small rocks asideand scoop up samples of Martiansoil. A few simple, automated ex-periments then analyzed the soil,searching for organic compoundsand signs of biological activity, suchas respiration. Although the exper-iments did show some surprisingresults, planetary scientists soon ex-plained these as chemical ratherthan biological reactions.

The negative results from Vi-king were surely disappointing tomany scientists, but the experimentsdid not rule out life on Mars. TheViking missions had, after all, sam-pled soil at only two relatively blandlocations on Mars, which had beenchosen because they made rela-tively easy landing sites, and thesearch was carried out by only afew simple experiments. But bud-getary and political considerations,the grounding of the space programfor three years after the tragic Chal-lenger accident in 1986, and the fail-ure of two Russian missions (Phobos 1 and 2) and one Americanmission (Mars Observer) to Mars all conspired to stop Martian explo-ration for some twenty years. The new era began with the landing of


(Top) This Viking orbiter photo shows dried-up ancientriverbeds on the Martian surface. (Bottom) This pro-

jection shows the Northern Hemisphere of Mars, withthe North Pole at the center. Darker areas represent

higher elevations. The black curves represent possibleshorelines of an ancient Martian ocean.

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Mars Pathfinder and its little rover, Sojourner, on July 4, 1997 (seeColor Plate 2).

Pathfinder, a simpler and considerably less expensive spacecraftthan Viking, landed on Mars in a rather innovative way. A parachuteslowed its descent through the Martian atmosphere, but it still hit thesurface at crash-landing speed. To protect itself, Pathfinder deployedairbags on the way down, which completely encased the spacecraftcomponents. These airbags allowed it to bounce along the surface untilit finally came to rest. Over the next several months, Pathfinder sur-veyed the landing site with cameras and other on-board instruments.Sojourner, which had been stowed on Pathfinder for the journey fromEarth, slowly roamed the neighborhood. (Sojourner was named forSojourner Truth, an African American heroine of the Civil War era whotraveled the nation advocating equal rights for women and blacks.)Although Sojourner could travel only a few meters from Pathfinder,this was enough to check the chemical composition of many nearbyrocks. The results confirmed what scientists had suspected: the land-ing site, in the Ares Vallis region, is a vast, ancient floodplain. Rocksof many types lie jumbled together as they were deposited by the flood,and the departing waters left rocks stacked against each other in thesame manner that floods do on Earth.

Pathfinder and Sojourner were designed more to test efficient andcheap new technologies than to search for life, but they marked thebeginning of an ambitious invasion of Mars by spacecraft fromEarth—and perhaps someday soon by human beings. As Earth andMars orbit the Sun, the two planets are sometimes reasonably close (if30 million miles can be called close) and other times much fartherapart. When they are close, which happens roughly every two years, a journey to Mars takes about eight months with current technology.

NASA hopes to send two or more space probes to Mars at everyone of these biennial opportunities over the next decade; however, thefailure of both 1999 missions (the Mars Polar Lander and the MarsClimate Orbiter) may slow the parade somewhat while scientists andengineers figure out how to prevent similar losses in the future. Other


nations, notably Japan and the member nations of the European SpaceAgency, also have plans for robotic missions to Mars. (The first Japa-nese mission to Mars, called Nozomi, was launched in 1998 but willnot arrive until at least 2003, due to an engine misfiring shortly afterlaunch.) The more sophisticated probes may carry rovers capable ofjourneying for many miles across the Martian surface in search of life.

Naturally, the search for life would be easier if we could bring asample of Martian soil back to Earth, where we could subject it to analy-sis more detailed than is possible with automated probes that remainon Mars. NASA hopes to launch a sample-return mission to Mars asearly as 2005, in which case we could have newly collected Martianrocks and soil in our hands as early as 2007 or 2008.

Surprisingly, that would not be our first sample of Mars, as scientistsbelieve that a few meteorites may have a Martian origin. Almost allmeteorites share common characteristics which tell us that they are ei-ther pieces of rock left over from the formation of our solar system orpieces of shattered asteroids. (Indeed, radioactive dating of the mostancient meteorites is what tells us that our solar system formed 4.6billion years ago.) But a few meteorites have unusual compositionsthat suggest a different origin. These rare meteorites seem to be chunksof rock from the surface of the Moon and Mars. Based on what weknow about the chemical composition of Mars from the Viking lan-ders, fifteen of these meteorites (as of this writing) have compositionsthat are near perfect matches for what we expect in rocks from Mars.

In 1996, a team of NASA scientists claimed that one of these Mar-tian meteorites, known as Allan Hills 84001, or ALH 84001 for short,contained evidence of past life on Mars. Meteorite ALH 84001 hadbeen scooped from the Antarctic ice by a team of meteorite hunters in1984. Careful study of the meteorite reveals its story. Radioactive dat-ing shows that the underlying rock formed from volcanic lava, pre-sumably on the surface of Mars, about 4.5 billion years ago. Sometimelater—probably about 3.6 billion years ago—the rock was heated anddeformed, so that a liquid was able to flow through it and depositsmall rounded globules of material. Much more recently, the rock washeated again, this time by the impact of an asteroid that blasted it off


the Martian surface. In space, the me-teorite was exposed to cosmic rays—high-energy particles that leave telltalechemical signatures on anything un-protected by an atmosphere. Carefulanalysis of ALH 84001’s cosmic ray ex-posure shows that it wandered throughthe solar system for at least 16 millionyears. Then, about thirteen thousandyears ago, it came crashing down toEarth, landing in Antarctica.

The globules found in ALH 84001contain chemical evidence suggestiveof life, including layered carbonateminerals and complex molecules (poly-cyclic aromatic hydrocarbons)—bothof which are generally associated withlife when found in Earth rocks. Evenmore intriguing, under high magnifi-cation the globules reveal eerily life-like, rod-shaped structures. However,while some scientists suggest that thesestructures might be fossils of micro-scopic Martian life, others argue thatall the so-called evidence for Martianlife in ALH 84001 could have beenproduced by nonbiological processes.

This extraordinary debate about whether we already have discoveredevidence of life beyond Earth will undoubtedly continue as scientistssubject ALH 84001 and other Martian meteorites to further scrutiny.Unfortunately, unless there is a surprising and major breakthrough, itis unlikely that the meteorites by themselves can provide definitive evi-dence of life. For that, we will have to rely on the planned armada ofspacecraft bound for Mars.


(Top) Meteorite ALH 84001 weighs a little less than 5 pounds and measures about 6 inches long by 4 inches by 3 inches. It is apparently a rock from

Mars. (Bottom) Some scientists believe that the rod-shaped structures seen in this microscopic view of a thin section from ALH 84001 are fossils of microbial

Martian life, but most others are skeptical.

Not long ago, most scientists would have said that Mars was notonly the best place to look for life in our solar system but the only place.Our Moon and the planet Mercury are both cratered worlds withoutatmospheres, and clearly could not harbor life as we know it. Venus isa near twin to Earth in size, but its thick carbon dioxide atmospherecreates a strong greenhouse effect that bakes its entire surface to 450°C(900°F)—far hotter than a pizza oven. In addition, the weighty atmo-sphere bears down on the surface with a pressure equivalent to thatnearly a kilometer beneath the ocean surface on Earth, and the Venu-sian clouds contain sulfuric acid and other corrosive chemicals. Suchconditions make it difficult to imagine life on Venus. With Mercury,Venus, and the Moon ruled out, and Mars already considered, we areleft with the worlds of the outer solar system. These worlds and theirmoons are magnificent, but they are very far from the Sun. WhereasMars orbits the Sun only about half again as far away as Earth, Jupiterorbits at five times the Earth’s distance, and the other outer planets(Saturn, Uranus, Neptune, and Pluto) are much farther still. Sunlightis too weak to provide the energy needed for life on these distantworlds. But who needs sunlight?

Until fairly recently, most scientists would have answered, “All lifeforms do.” But the past few decades have seen extraordinary discover-ies about the abodes of life on Earth. Biologists have found microor-ganisms living deep inside rocks in the frozen deserts of Antarcticaand inside rocks buried nearly a mile underground. Perhaps most im-portant to the search for life beyond Earth, scientists have found teem-ing life near deep undersea volcanic vents. This life does not in anyway depend on sunlight, instead receiving all its chemical energy fromthe heat of the Earth itself. Moreover, genetic analysis of life-formsaround the volcanic vents suggests that they may be more closely re-lated to the common ancestor of all life on Earth than any other livingorganisms. Many biologists therefore believe that life may have firstarisen around volcanic vents, with sunlight-dependent life comingonly later. If so, then the water around undersea volcanoes might be


the best place to find life in our solar system—and we know of at leastone place beyond Earth where such volcanoes may be found.

Europa is one of four large moons that orbit Jupiter, along with adozen smaller moons. Europa was first seen through a telescope byGalileo, but we had no close-up pictures of it until the Voyager 1 and2 flybys in 1979. Photographs of Europa show a surface with very fewimpact craters. In a solar system filled with rocky debris, a nearly crater-free surface can mean only one thing: some process is continually eras-ing the evidence of impacts. Because Europa’s surface is made almostentirely of water ice, it seems likely that the craters have been erasedby flowing water. Europa is therefore thought to have a huge sub-surface ocean beneath its icy crust. Recent, detailed photographs taken by the Galileo spacecraft—which has been orbiting Jupiter since


(Left) A global view of Europa shows a fracturedcrust nearly devoid ofcraters. (Below) This close-upof Europa’s surface showsjumbled crust in which ice-bergs are apparently frozenin slush. (Photos taken fromNASA’s Galileo spacecraft.)

1995—support this basic scenario. Moreover, the structure of the sur-face ice suggests at least occasional heating from below, presumably by volcanoes sprouting from the floor of Europa’s ocean (see ColorPlate 3).

With a deep ocean and a source of energy in its undersea volca-noes, Europa may be even more likely to harbor life than Mars. In-deed, if biologists are correct in guessing that life on Earth first evolvednear undersea volcanic vents, an absence of life on Europa may be moredifficult to explain than its presence.

Not surprisingly, Europa is now a prime target for scientific explo-ration. NASA is currently developing plans for a Europa orbiter thatcould be launched as early as 2003 to reach Europa in 2006 or 2007.This mission would carry a radar instrument designed to establishwhether or not Europa really has a subsurface ocean, and perhaps howmuch liquid water lies beneath the icy crust. If the existence of anocean is verified, a follow-on mission would send a spacecraft to landon Europa, where it would use battery-generated heat to melt its waythrough the crust and into the ocean. There’s no telling what such amission might find, from microbial life to creatures larger than whales.

Europa provides the clearest case to date for a subsurface ocean,but Ganymede and Callisto, two other moons of Jupiter, may also havesuch oceans. These worlds, too, have icy surfaces, but their larger num-bers of craters show that water flows occur less commonly, if at all.Nevertheless, it’s certainly worth looking for life on both of these moonsas well. Imagine the irony if we found life on all three. There might bemore inhabited worlds around the far-flung planet Jupiter than in therest of the solar system combined.

Hopes for finding life dim beyond Jupiter, but a few scientists holdout the possibility of life on Saturn’s moon Titan. Although we usuallythink of moons as being airless, Titan has a thick atmosphere—likeEarth’s, it is made mostly of nitrogen. The surface pressure is onlyslightly greater than that on Earth, which means that it would be fairlycomfortable if not for the lack of oxygen and the bone-chilling tem-peratures. Titan is far too cold for liquid water today, but the heat ofimpacts early in its history might have created slushy ponds in which


life potentially could have arisen. If life did arise, perhaps some organ-isms survive to the present day in geological hot spots or somehowadapted to survive despite Titan’s icy temperatures. We do not yet knowwhat Titan’s surface looks like, because the thick atmosphere hides itfrom view. However, the combination of atmospheric compositionand surface temperature, both of which have been measured, leadsmany scientists to believe that Titan may be partially or fully coveredby a deep ocean of liquid ethane.

Answers about Titan may be coming fairly soon, because a space-craft called Cassini is scheduled to arrive there in 2004. Cassini waslaunched in 1997 and carries a radar instrument that should allow usto map Titan’s surface in detail. It also carries a probe called Huygens(named for the seventeenth-century astronomer who discovered Titan),which it will drop to the surface of Titan. Just in case, the probe is de-signed to float in liquid ethane.

We have now identified five places in our solar system with a rea-sonable chance of harboring Earth-like life: Mars, Europa, Ganymede,Callisto, and Titan. If we allow for less likely locations, the list expandseven more. The four giant planets of our solar system—Jupiter, Saturn,Uranus, and Neptune—have no solid surfaces upon which to look forlife, but their atmospheres are rich in the chemical building blocks oflife, and at some depths the temperature would be quite comfortablefor life. Although it is difficult to imagine how life might arise in theair in the first place, once it got started life might survive just fine—aslong as it found some way to stay at a comfortable depth despite theturbulent atmospheric currents.

A more speculative idea suggests that life might exist on comets,which we know to contain large amounts of organic material. A fewscientists have even suggested that life on Earth may have been broughthere by a comet. If so, comets may have seeded life on many otherworlds. Moreover, the presence on Earth of meteorites from Mars tellsus that planets occasionally exchange rocks dislodged by major impacts.The harsh conditions under which some life on Earth exists suggestthat living organisms might survive such impacts and the long journeyfrom one planet to another. In that case, life may have migrated among


the planets just as various species spread among islands on floatingdebris. Life from Earth may have made its way to Mars—or vice versa.If we ever discover life on another world, we should be able to tellwhether it shares a common origin with life on Earth by examining itsset of genetic instructions.

The search for life may well be a never-ending quest. If we don’tfind it, we can always believe that we simply haven’t yet looked hardenough or in the right places. But whether or not we find life, the ex-ploration of Mars, Europa, and Titan planned for the next decadeshould provide deep insights into the conditions under which life cansurvive. And if this coming exploration does find life on other worlds,our Mystery 10 will be solved before the third millennium is barelyunder way.


The inset (right) is a photograph of part of Titan from Voyager 2.Note that it is completely enshrouded by its thick atmosphere.The artist’s conception (above) shows what the surface might

look like as the Huygens probe descends in 2004. Saturn andthe Cassini orbiter are faintly visible through the atmosphere.(Artistic license has been taken to make the orbiter look big

enough to be seen from the moon’s surface.)

reality provides us with facts so romantic that imagination itself … · 2007-12-26 · sojourner...

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